In the drilling of oil and gas wells it is common practice to case and cement the wellbore. This operation is typically conducted in several stages. An initial section of the well is drilled with a large diameter bit. A large diameter tubular casing is then set within this initial section of the well. Cement is then pumped through the bottom of the casing under sufficient pressure to cause it to flow upward and fill the annulus defined by the wellbore and the casing. As the cement is pumped, it displaces to the surface of the well the drilling fluid which fills the annulus at the time the casing is set. After the cement cures, a smaller diameter drill bit is passed through the first section of the casing and a second section of the well is drilled, cased and cemented. This process continues until the desired depth is reached.
A principal reason for casing and cementing a well is to establish a seal which prevents fluid communication among the various rock strata traversed by the wellbore. Occasionally, portions of the drilling fluid within the annulus are bypassed in the cementing operation. This bypassed drilling fluid most typically takes the form of channels extending a considerable distance along the annulus. This condition is called "channeling." FIG. 1 shows a cross-section of a cased borehole 10 in which channeling 12 has occurred in the cemented annulus 14. Channeling has the effect of establishing pathways along the annulus through which fluids can pass. This can result in the loss of hydrocarbons to "thief" strata; permit the production of unwanted fluids; decrease the effectiveness of acidizing, formation fracturing and other reservoir stimulation treatments; and can prevent injected drive fluids from efficiently displacing hydrocarbons in enhanced oil recovery operations. In many instances channeling, if uncorrected, can render a well worthless.
Channeling and similar cementing problems are typically corrected by a remedial cementing operation known as "squeeze cementing." In squeeze cementing, the casing is perforated at the location of the channel and cement is forced through the perforations into the channel under high pressure. The fluid within the channel is displaced into the formation by the cement. Though squeeze cementing is generally not expensive to perform, the delays in completing a well owing to the need for remedial cementing often result in significant economic loss.
It is common practice to perform a logging operation known as a "cement bond log" on newly cemented sections of a well to determine the adequacy of the cement job. Where the cement bond log indicates that channeling may have occurred, the driller must determine whether remedial cementing should be conducted. This determination is complicated by the fact that the interpretation of cement bond logs is an inexact science. An indication of a cementing problem on a cement bond log could be the result of channeling, which can be corrected by remedial cementing, or could be the result of defective cement, which generally cannot be corrected by remedial cementing.
It would be desirable were there a method for examining regions of a cased wellbore which are suspected to have an imperfect cement seal to conclusively determine whether channeling has occurred and, if so, the size of the channel. Several methods have been proposed for this; however, none are commercially available.
U.S. Pat. No. 2,451,472, issued Oct. 19, 1948, teaches the use of a drilling fluid containing a radioactive tracer for the purpose of permitting an accurate determination of the location and magnitude of channels. After the cementing operation, a gamma log of the well is conducted to detect the presence of any remaining drilling fluid. Though this method is accurate and inexpensive, the required use of significant amounts of radioactive material renders it impractical for most applications.
U.S. Pat. No. 2,451,520, issued Oct. 19, 1948, teaches a channeling detection method in which critical intervals of a cased wellbore (e.g. those near hydrocarbon bearing formations) are perforated and a liquid doped with a radioactive tracer is pumped through the perforations. The radioactive tracer will fill any channels, which can then be detected by subsequent gamma logging. Though this method yields accurate results, it is undesirable due to the expense of the perforating and injection operations required.
It would be desirable to develop a method for establishing the location and quantity of bypassed drilling fluid in a cased wellbore quickly and accurately without the need for injecting radioactive tracers into the well or drilling fluid.